KR20070012708A - A process for the production of polylactic acid(pla) from renewable feedstocks - Google Patents

Abstract

The present invention relates to an efficient process for producing polylactic acid from fermentation of renewable agricultural feed-stocks not limited to molasses or cane bagasse employed as starting material. The present invention in particular provides a cost effective and industrially scalable process for producing polylactic acid obtained by fermentation of Lactic acid having industrial applications. ® KIPO & WIPO 2007

Description

Production method of polylactic acid (PLA) from renewable raw materials {A PROCESS FOR THE PRODUCTION OF POLYLACTIC ACID (PLA) FROM RENEWABLE FEEDSTOCKS}

This application claims the priority of the Indian Patent Application of Application No. 576 / MUM / 2004, filed May 2004.

The present invention relates to an efficient process for producing polylactic acid by fermenting renewable agricultural raw materials, which are not limited to molasses or sugarcane bagasse used as starting materials. In particular, the present invention provides a cost-effective and industrially scalable method for producing polylactic acid obtained by fermentation of lactic acid with industrial use.

Lactic acid fermentation has gained increasing attention in recent years mainly because of its importance as a building block in the manufacture of biodegradable plastics. Lactic acid can be produced from a variety of substrates, such as whey membrane permeate and starch hydrolysate, which are sources of lactose and glucose, respectively.

Recently, the potential of lactic acid as a starting material for a new class of renewable biodegradable lactide polymers has been recognized. These biodegradable polymers are considered as alternatives to current plastic materials, or biodegradable products such as support and adhesion membranes used in bone surgery, useful plastic products and containers, medical garments, disposable diapers, yard bags, etc. It is considered for a variety of new uses in their development.

Various purity or grades of lactic acid are used for different applications. Low technical grades of lactic acid are used for technical purposes such as in metals and leather, and slightly refined food grade lactic acid is used in food-related applications. In pharmaceutical applications, high purity grades are used. On the other hand, when lactic acid is used as the starting material for the lactide polymers, very pure and thermally stable grades, especially the free isomer L- or D-forms, are preferred. India is rich in renewable agricultural resources that can be used as a raw material for the production of lactic acid, the building block for the production of polylactic acid (PLA).

Polylactic acid (PLA) is a biodegradable polymer derived from lactic acid. It is a very versatile material and is made from 100% renewable resources such as corn, sugar beets, wheat and other starch rich products. Polylactic acid exhibits many properties that are equal to or better than many petroleum-based plastics, making polylactic acid suitable for a variety of applications. Polylactic acid is a multipurpose polymer with many potential uses, including many uses in the textile and medical industries as well as the packaging industry. Polylactic acid also has many potential uses in the textile and nonwoven fields. It is easily converted into various fiber forms using conventional melt-spinning processes.

Known worldwide petroleum resources are estimated to be depleted within 80 years, natural gas within 70 years, and coal within 700 years, but the economic impact of such depletion could come much sooner due to soaring prices from resource depletion. have. It is clear that researchers need to study fossil fuel resources as a renewable resource for many oil-based products. Progress is being made using polymers called polylactic acid (PLA), which are affordable, recyclable, and innovative materials made from renewable resources. Production of polylactic acid from renewable agricultural raw materials is an attempt to balance resources and make plant-based substitutes for fossil fuels. In addition, the good biodegradability and recyclability / compostability of the lactic acid based polymers further increase their potential and development needs.

The present invention attempts to produce polylactic acid from renewable agricultural raw materials such as starch materials, wood, sugar cane bagasse, straw, cellulosic materials such as rice straw, and molasses derived from sugar cane or sugar beet. Provide an efficient way.

Lactic acid can be produced commercially by chemical synthesis or renewable carbohydrate fermentation. In response to increased public interest in greenhouse gas emissions and environmental pollution and government regulations, lactic acid produced by environmentally suitable fermentation biological processes using renewable biomass is fossil fuels (coal, petroleum, or natural gas). It is preferable to chemical synthesis using.

Polylactic acid is not a new substance. It has existed for decades. In 1932, DuPont scientist Wallace Carothers produced lactic acid products by heating lactic acid under vacuum. In 1954 DuPont patented a further refined Carothers process.

Because of the high cost, subsequent focus has mainly focused on the manufacture of medical grade suture chambers, implants, and controlled drug release applications. The cost of monomer production hindered the widespread development of the polymer. Recently, progress has been made in fermenting glucose to convert glucose to lactic acid. This dramatically lowered the production cost of lactic acid and significantly increased interest in polymers.

Lactic acid, ie 2-hydroxy propionic acid or α-hydroxy propionic acid, is produced / produced by synthetic and fermentation methods, is used in food preservation, pharmaceuticals, leather tanning and metal pickling and is a starting material in special chemical processes. The two optically active isomeric (enantiomer) forms of lactic acid are indicated as L (+) or S (+)-priorities and D (-) or R (-)-left linearities, shown below.

Chemical synthesis of lactic acid results in racemic mixtures in which both enantiomers are present in equal proportions, and microbiological processes predominantly produce one of the enantiomers.

L-lactic acid is needed to produce biodegradable polymers. The production of L-lactic acid is usually accomplished by selecting suitable microbial strains in the fermentation process. It includes the conversion of monosaccharides such as glucose, fructose, galactose, or disaccharides such as sucrose or lactose to lactic acid. Some homogeneous strains producing only lactic acid as products are Lactobacillus delbruekii , L. casei , L. acidophilus and L. bulgaricus . The former consumes sucrose, glucose or fructose but not lactose, and the latter three species consume lactose and galactose in addition to the other sugars. Biological production of lactic acid is complicated by inhibition due to the pH drop due to lactic acid production and expensive downstream processing for producing lactic acid from dilute aqueous fermentation broth. A conventional method for producing lactic acid is anaerobic fermentation by Lactobacillus species in a batch reactor. To maintain the fermentation process, the acid produced is neutralized with alkali or removed from the fermentation system. Conventional processes use calcium carbonate or sodium hydroxide to neutralize the acid produced. The corresponding lactate is then separated from the broth by various processes including solvent extraction, electrodialysis or distillation, or a combination of one or more processes.

The production of polylactic acid requires high purity lactide obtained from the predominant L-lactic acid biochemically produced from renewable edible sources such as carbohydrate raw materials. Conventional industrial processes include the removal of biomass from fermentation broth followed by acidification, purification, concentration and polymerization. The present invention describes an efficient way in which L-lactic acid is predominantly produced from fermentation of cheap renewable agricultural raw materials, such as molasses or sugarcane bagasse hydrolysates, wherein such raw materials are simultaneously separated, purified and concentrated to allow crude lactide. Which is further purified and then polymerized with polylactic acid.

Fermentation production of lactic acid is a commonly used method to obtain optically pure isomers for the production of polylactic acid. U.S. Patent No. 1, which is authorized by Carlson et al 6475759 (November 5, 2002) provides low pH lactic acid fermentation, which includes culturing acid resistant homolactic acid bacteria in a nutrient medium to produce fermentation broth with high levels of free lactic acid. This patent specifically relates to bacteria capable of withstanding low pH, and therefore it is limited to processes using specific bacterial strains. The fermentation process used here is in batch form. Furthermore, this prior art neutralizes the acid produced using calcium carbonate, which ultimately produces large amounts of calcium sulfate (gypsum) which may have waste disposal problems, which is a undesirable environmental target. Is considered.

Corn syrup, starch, corn-steep liquor, corn oil, whey, sugar, sugar beet and sugarcane juice are commonly used as raw materials for the fermentation process. European Patent No. 0393818 (Glessner, David A., et al.) Provides a process for the production and purification of lactic acid, which is responsible for lactic acid producing microorganisms until most of the carbohydrates are converted to lactic acid in inexpensive substrates containing carbohydrates, corn steep liquor and corn oil. Growing stage. All of these materials are valuable as foods. In fact, the cost of the substrate is one of the challenges in the cost effective production of lactic acid by fermentation. The inventors of the present invention used inexpensive substrates of molasses and sugarcane bagasse hydrolysates, by-products / waste of the sugar industry, which have no value as food, as raw materials for lactic acid fermentation.

The above-mentioned patent contains a process for fermenting using healthy strains of lactate producing microorganisms that produce high concentrations of lactate salts from low-cost fermentation medium, and containing cellular and nitrogenous impurities using conventional electrodialysis. Lactate is recovered and concentrated from the whole broth, and the lactic acid is converted into lactic acid obtained by water decomposition electrodialysis and converted into lactic acid, followed by a base treatment with an ion-exchanger to remove charged impurities from lactic acid. Relates to the production and purification process. This method is limited in that it uses expensive electrical energy rather than other energy sources to proceed with the process. Moreover, the polymer membranes used in the electrodialysis process are very sensitive to impurities, so their application to the fermentation products of molasses requires expensive purification operations.

European Patent No. 0790229A1 (Donald McOulgg et al.) Describes a process for recovering lactic acid from a medium by contacting the medium with a solid-phase free base polymer having a tertiary amine group or a pyridine group capable of absorbing lactic acid. Lactic acid is then desorbed using strong acid or hot water. Filling of resins by other undesirable ionic and acidic species in fermented broth, requirements for high regeneration efficiency, and contamination of these resins by pigments, which are large organic molecules present in the broth, are the major limitations of this process.

U.S. Patent No. 6,569,989 to Ohara et al., US Pat. 6,326,458 (Gruber et al.) And WO 93/00440 (Michael Cockrem et al.) Relate to a process for producing lactide and polylactic acid from lactic acid obtained by fermentation, which synthesizes lactate esters from lactic acid, distills lactate esters and , Polycondensation of the lactate ester in the presence of a catalyst to obtain a prepolymer having a molecular weight in the range of 5000 to 15000, depolymerization of the prepolymer to obtain a lactide, polycyclic acid is obtained by ring-opening polymerization. If desired, the ester is hydrolyzed to produce lactic acid. This process involves two stages of energy intensive distillation, with the potential for racemization.

U.S. Patent No. 6,472,559 to Avraham et al., European Patent No. 0804607B1 (Abraham. M. Baniel, et al.) And US Pat. 6,087,532 (Abraham. M. Baniel) describes a process for separating and recovering lactic acid from fermentation broth using mixed solvents in the presence of CO ₂ and back-extracting with water at elevated temperatures between 80 ° C and 240 ° C. This process generally involves preconcentrating the raw material solution to remove water to levels between 40% and 70%, which is energy intensive. This process has limitations such as the use of expensive solvents such as high molecular weight trialkyl amines, the difficulty in handling solvent mixtures, their recovery and other related problems.

U.S. Patent No. 5,310,865 (Katashi Enomoto et al.) Discloses a process for preparing polyhydroxy carboxylic acids by carrying out dehydration of hydroxycarboxylic acids or oligomers in a reaction mixture containing an organic solvent. Organic solvents are used to remove water from condensation by azeotropic distillation. The preparation of very high molecular weight polymers of more than 1,00,000 required in many applications is one of the limitations of this process given the difficulties associated with the removal of trace moisture.

U.S. Patent No. A continuous process for the preparation of lactide and lactide polymers disclosed in 6,326,458 (Patrick Richard Gruber et al.) Produces lactide and lactide polymers from esters of lactic acid or lactic acid in the presence of a catalyst, thereby preparing crude polylactic acid, prepolymers. And in the case of esters, obtaining reaction byproducts. Crude lactide obtained from the prepolymer is purified by distillation again before use in the polymerization. By-product generation and crude concentration of crude lactide distillation when using lactic acid ester as starting material is a limiting factor that substantially adds to the cost of this process.

Prior art methods for the production of lactic acid by fermentation all require relatively pure substrates for bacterial growth and / or complexes and separation of lactic acid and toxic solvents. In addition, it is difficult to recover the solvent, and high energy is used to separate such solvent by distillation. Thus, these processes are expensive and time consuming. Some prior art provides low pH lactic acid fermentation comprising culturing acid resistant homolactic acid bacteria in a nutrient medium to produce fermented gravy having high levels of free lactic acid. Furthermore, the prior art uses calcium carbonate to neutralize the produced acid, which ultimately produces a large amount of calcium sulfate (gypsum), which solves the waste disposal problem because calcium sulfate is considered an undesirable environmental target. I can have it.

In view of the current urgent needs, the scientists have developed a new method of producing polylactic acid from fermentation of renewable non-edible agricultural raw materials.

According to the present invention, polylactic acid is produced from fermentation of renewable non-edible agricultural raw materials, the process being relatively inexpensive compared to conventional processes using starch substrate as raw material.

In this method, pH adjustment is performed using ammonia, which does not form salt precipitate (gypsum) as in the conventional process. Since the process does not have salt precipitate formation, the present invention solves the problem related to the waste disposal problem.

Compared with conventional processes, compared to the use of a mixture of expensive solvents and preconcentration steps in some conventional processes, the present invention provides a method for separating lactic acid from fermented broth using a single bulk solvent at ambient temperature. .

Furthermore, the present invention provides a nearly quantitative method in this process without any energy step of regeneration / recovery of the solvent at high temperature operation and / or distillation. Thus, the regenerated solvent is recycled for extraction without any further treatment.

In this method the prepolymerization and lactide forming steps are combined in a single unit operation. In addition, the present invention provides a process wherein separation, concentration and purification are achieved simultaneously using an affinity propulsion process at ambient temperature instead of the energy propulsion process used in some conventional processes.

The method developed by the inventors is efficient, cost effective and less cumbersome compared to conventional processes.

Purpose of the Invention

It is an object of the present invention to provide an efficient method for producing polylactic acid from renewable agricultural raw materials as starting materials. The extraction efficiency in the process of the present invention exceeds 90% over the 90% claimed by conventional processes.

The object of the present invention is an efficient method of producing polylactic acid from renewable agricultural raw materials such as molasses and sugarcane bagasse hydrolysates, which are waste products of the sugar industry, in the most efficient manner and thus very cost-effectively for producing PLA. To provide.

Another object of the present invention is to provide a method which can be successfully applied to other renewable agricultural raw materials such as corn syrup, sugar beet, sugar cane juice.

Another object of the present invention is to provide a method for separating lactic acid from fermented broth using a single bulk solvent at ambient temperature as compared to the use of a mixture of expensive solvents and preconcentration of fermented broth in some other processes.

It is a further object of the present invention to provide a nearly quantitative method in the process of the invention without any energy step of regeneration / recovery of the solvent at high temperature operation and / or distillation.

Another object of the present invention is to provide an affinity-propelled, less energy-concentrating method that utilizes hydroxides of alkali or alkaline earth metals in the back-extraction step as compared to the temperature propulsion and energy concentration operations of some other processes. .

Another object of the present invention is to provide a process for forming lactide using a concentrated lactic acid solution directly in a high temperature steam reactor at 120 ° C. to 155 ° C., wherein the corresponding liquid temperature is 180 ° C. to 225 at a vacuum of 5 to 30 mm Hg. ° C, whereby the prepolymerization and lactide forming steps are combined in a single unit operation.

Another object of the present invention is to provide a method for purifying lactide by crystallization with an organic solvent, wherein the mother liquor is recycled and used for further preparation of lactide. Some other processes disclose purification by vacuum distillation involving very high vacuum and energy.

Another object of the present invention is to provide a method of simultaneously separating, concentrating and purifying using an affinity propulsion process at ambient temperature instead of the temperature or electric propulsion process used in some other processes.

The above and other objects and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments in conjunction with the accompanying drawings.

1 is a process chart showing the production of polylactic acid from fermentation of molasses according to one embodiment of the present invention.

Detailed description of the invention

The present invention describes a method for producing polylactic acid using renewable agricultural raw materials such as molasses and sugarcane bagasse hydrolysate raw materials. The renewable agricultural raw materials used may be edible or non-edible. The process according to the invention comprises the fermentation of the raw material to lactic acid and the separation of it from the fermentation broth and conversion to lactide and polylactic acid. The method according to the invention comprises the following steps:

i. Lactic acid medium was prepared using sugarcane molasses or sugarcane bagasse as a raw material;

ii. Fermentation of lactic acid;

iii. Separation of lactic acid; And

iv. Preparation of Lactide and PLA.

In one embodiment of the invention, the fermentation medium is prepared using sugarcane molasses or sugarcane bagasse as raw material.

In the present invention, the fermentation medium is prepared using sugar cane molasses containing about 40% to 50% (w / w) of fermentable sugars, and centrifuged to remove suspended material and diluted with water, The final concentration is 7% to 10% (w / w). This medium is supplemented with other nitrogen sources and other growth components. The diluted molasses solution in a large scale process is fortified with 2% to 5% corn steep water and autoclaved yeast paste.

Fermentation medium using sugarcane bagasse as a raw material is prepared by hydrolyzing bagasse particles with dilute acid at 120 ° C. for 30 to 90 minutes. The hydrolyzed bagasse is delignified by further treatment with 1% to 5% (v / v) alkali at 120 ° C to 150 ° C for 15 to 90 minutes. The cellulose portion thus obtained is enzymatically hydrolyzed with a cellulase enzyme. Glucose-rich sugarcane bagasse hydrolyzate is supplemented with a nitrogen source prior to fermentation.

According to the invention, the fermentation of lactic acid is carried out in a batch manner for 24 to 48 hours under anaerobic / aerobic conditions using strains of bacteria such as Lactobacillus . In a preferred embodiment according to the invention, the Lactobacillus strain used is L. delbrueckii .

According to the invention, the pH of the fermentation medium is easily obtained by the pH adjusting method using a neutralizing agent. Neutralizers that can be used according to the invention to maintain a suitable pH level are selected from alkali or carbonate or ammonia solutions. During fermentation the pH of the medium is kept constant between 5.0 and 6.0 using alkali or carbonate or ammonia solutions. Fermentation temperature is maintained at 37 ℃ to 45 ℃. After the fermentation is complete, the broth containing lactic acid is centrifuged to clear and further downstream processing is performed.

The cleared fermentation broth is extracted using an organic solvent such as isoamyl alcohol, butanol, cyclohexanone, methyl acetate, etc. in a ratio ranging from 1: 1 to 1: 5 using a suitable contact device such as a rotary disk contactor. The extract is taken for further treatment. The extraction efficiency obtained by the present invention is 90% or more. The lactic acid containing extract is contacted with an aqueous alkali such as ammonia, lime water, caustic and the like in a ratio of 1: 3 to 1:10. The mixture is stirred for 1 hour and left for 2 hours to allow phase separation. The aqueous phase containing the lactic acid salt is separated with an organic extractant and taken up and subjected to downstream treatment. The organic layer is recycled to the extraction process. The aqueous phase containing the lactic acid salt is treated with an inorganic acid to separate the lactic acid and, if a salt is formed, collected by centrifugation. The crude lactic acid thus obtained is treated with a cationic resin to remove any trace metal ions and concentrated at least 5 to 6 times by vacuum distillation.

In another aspect of the invention, the concentrated lactic acid is converted to lactide in a jacketed stirred tank reactor equipped with vacuum and heating equipment. Initially, the contents are heated to 120 ° C. to 150 ° C. to remove water, and then the catalyst is added in the range of 0.05% to 2.0% (w / w). The reaction is continued for another 5 to 24 hours, with the temperature gradually increasing from 140 ° C. to 275 ° C. and the vacuum gradually increasing to 1-20 mm Hg. After the remaining water is removed, the formed lactide is distilled off and collected in a cooled collection vessel.

The obtained lactide is further purified by crystallization with an organic solvent such as ethyl acetate, toluene or alcohol. Purified lactide is polymerized in the presence of a suitable catalyst in a stirred tank reactor. The temperature is maintained at 120 ° C. to 200 ° C. for 1 to 20 hours. The resulting polymer, ie polylactic acid (PLA), is dissolved in a suitable solvent and precipitated in another solvent used as flocculant to characterize it. The resulting product is dried and characterized.

The inventors of the present invention find that the method of the present invention differs from previously disclosed processes or known techniques for the production of PLA in that the method of the present invention effectively uses cheap waste in the sugar industry for fermentation of lactic acid and recovery from fermentation broth. Developed. Furthermore, the present invention provides a process for concentrating and purifying lactic acid for further downstream processing for the production of lactide and PLA. The approximate concentration of lactic acid in the fermentation broth ranges from about 4.5% to about 5.0% w / v, which is 30 minutes to 1 hour in an LLE device such as a mixer-precipitator, filling column, rotary disc contactor, or pulsation extraction column. Extraction with suitable organic solvents such as alcohols, ketones, esters and ethers, with sufficient residence time in the range and volume ratios of 1: 1, 1: 1.25, 1: 1.5, 1: 2, 1: 3, 1: 4, etc. This results in good phase separation between the raffinate (aqueous layer) and the extract (organic layer), resulting in an extraction efficiency of at least 90%.

Next, a solvent extract phase containing any impurity that adds color to the lactic acid and the extract is subjected to a known amount such as sodium hydroxide, calcium hydroxide, ammonium hydroxide, etc., with proper stirring in a conventional stirrer vessel for 30 minutes to 1 hour. Treatment with alkali or alkaline earth metal hydroxides is followed by standing for an additional time for good phase separation. The aqueous layer containing lactic acid is removed and subjected to downstream treatment. The extract (organic layer), ie the regenerated solvent, is removed and then recycled back to the LLP apparatus and reused as mentioned above.

The aqueous phase containing the salt of lactic acid called lactate is then used as sulfuric acid at a pK value of lactic acid, i.e. at a pH below 3.86, more preferably at a pH of less than 3, most preferably at a pH of 2.5 to 3.0. Treatment with inorganic acid precipitates metal salts. The acidified lactate solution is then centrifuged to remove the metal salt in the form of a cake, which is then thoroughly washed with demineralized water to extract the absorbed lactic acid, if any, and then recycled to the metal precipitation step. Next, the filtered aqueous centrifugation / filtration liquid is taken and subjected to further downstream treatment.

Next, metal ions remaining in dilute lactic acid in concentrations ranging from 9% to 10% w / v are removed while adjusting the inlet and outlet flows to a suitable flow rate in a continuous column packed with ion-exchange resin. The end point of the column treatment operation is indicated by measuring the pH value or metal ion directly.

The diluted lactic acid solution is then concentrated to a level of 50% to 60% w / v in a packed distillation column at a suitable reflux ratio in the range of 1: 3 to 1: 5 at a vacuum of about 300 to 350 mm and a temperature of 75 to 85 ° C. do. The collected distillate, mainly water, is then recycled to the precipitation step. The concentrate at the bottom of the boiler of the distillation column is then sent to the lactide production stage.

The concentrated lactic acid is then placed in a suitable capacity stirred tank reactor equipped with a suitable constituent material, namely a stirring and heating / cooling facility, and a downstream condenser of suitable surface area. Use of a suitable catalyst at a vapor temperature of 120 ° C. to 155 ° C. and a liquid temperature of 190 ° C. to 225 ° C. under a vacuum of 10 mm to 30 mm Hg and removal of free and bound water during the course of the reaction yields the necessary details of lactide. . The residue of the autoclave contains unconverted lactic acid, trimers and other high boiling components. The average yield of lactide is 75% to 80% of the theoretical amount.

The crude lactide thus obtained is further purified by crystallization with organic solvents such as ethyl acetate, ethanol, methanol, butanol and a group of other solvents. The crystallized lactide is then filtered off. The purified lactide is further recrystallized with any solvent to completely remove the coloring material. The yield of purified lactide is in the range of 50% to 60%. The mother liquor can be reprocessed / recycled to produce more lactide.

Next, the purified crystalline lactide was added to the calcium, zinc or tin compound in a stirred tank reactor with heating / cooling coils under a suitable temperature profile that varied from 185 ° C to 110 ° C with proper stirring for about 15-20 hours. The polymerization is carried out in the presence of a suitable catalyst based on the catalyst. The resulting polymer product (PLA) is then dissolved in a suitable solvent, precipitated and dried in another solvent which acts as a flocculant. Polymers of molecular weight ranging from 20,000 to 1,00,000 are obtained that can be used in a variety of applications.

According to the present invention, fermentation can be carried out using microbial cells of suitable strains which produce various metabolites. In some embodiments according to the invention for the production of lactic acid, the microbial cells usable are selected from bacteria such as Lactobacillus, Streptococcus, Bacillus or strains of fungi such as Rhizopus . Different strains belonging to Lactobacillus used according to the invention for lactic acid production are L. delbrueckii, L. rhamnosus , L. helveticus , L. casei , L. plantarum , L. bulgari- cus, L. amylovorans , L. lactis, and the like. Can be selected from. The Lactobacillus strain used in the preferred embodiment according to the invention is L. delbrueckii .

According to the present invention, the fermentation can be any of a continuous fermentation process, a batch fermentation process or a fed-batch fermentation process. Preferably, the method comprises culturing the bacteria in a batch fermentation process.

The operational flow diagram of the above-described method of producing PLA or salts thereof from molasses is shown in FIG. 1.

The present invention regarding a multi-step process chain for producing PLA from molasses fermentation has the advantage over other processes with respect to:

1. This method is very cost effective because PLA is produced in the most efficient manner using renewable agricultural raw materials called molasses, which is a waste of the sugar industry. The method can be successfully applied to other renewable agricultural raw materials such as corn syrup, sugar beet, sugar cane juice, which are relatively clean compared to molasses in terms of contamination.

2. The above-mentioned method uses a single bulk solvent for the separation of lactic acid from fermentation broth at ambient temperature as compared to the use of a mixture of expensive solvents and a preconcentration step in some other processes. In comparison with the 90% claimed by the other processes, the extraction efficiency in this method was at least 90%.

3. In this method, the regeneration / recovery of the solvent is continuous and almost quantitative, without any energy step of high temperature operation and / or distillation.

4. The back-extraction step uses hydroxides of alkali or alkaline earth metals and is affinity-propelled and less energy concentrated compared to the temperature propulsion and energy concentration operations of some other processes.

5. The concentrated lactic acid solution is used directly to form the lactide in a hot steam autoclave of 120 ° C. to 155 ° C., where the corresponding liquid temperature is 180 ° C. to 225 ° C. in a vacuum of 5 to 30 mm Hg, thereby prepolymerizing and locking The tide forming steps are combined into a single unit operation.

6. Purification of lactide is carried out by crystallization with an organic solvent, and the mother liquor is recycled and used for further preparation of lactide. Some other processes disclose purification by vacuum distillation involving very high vacuum and energy.

7. Instead of the temperature or electric propulsion process used in some other processes, separation, concentration and purification are simultaneously achieved using an affinity propulsion process at ambient temperature.

The invention will now be illustrated by some embodiments. These examples illustrate how the invention is practiced and do not limit the invention. Most of the laboratory work was done using laboratory setups / devices. Several steps of the method were performed using a continuous or batch laboratory scale, bench scale, or pilot scale device as a single process step / unit operation, which demonstrates the feasibility of using such process technology on a commercial scale. .

Example  One

Lactobacillus delbreuckii was cultured for 24 hours in 250 mL of fermentation medium containing sugar cane molasses adjusted to a final fermentable sugar concentration of 70 g / L and fortified with corn immersion water (25 g / L) and yeast extract (10 g / L). It was added as an inoculum of 10% concentration. Fermentation was carried out in a shake flask of Gallenkamp culture shaker at 150 rpm, 42 ° C. for 72 hours. The pH of the fermented broth was maintained at 6.0 using CaCO ₃ . After 72 hours, the yield of lactic acid was 50 g / L.

Example  2

A 2 L batch of fermentation medium containing sugarcane molasses and supplemented with 25 g / L corn steep water and 10 g / L yeast extract was run in a 2.5 L bioreactor. The initial sugar concentration was adjusted to 70 g / L. 200 ml of Lactobacillus delbreuckii cultured for 24 hours was inoculated into the medium. Fermentation was carried out at 42 ° C. for 48 hours with stirring at 200 rpm. The pH of the medium was maintained at 6.0 using liquid ammonia. After completion of the operation, the yield of lactic acid was 66 g / L.

Example  3

The lactic acid fermentation scale was increased to a 500L fermenter with a working volume of 350L. L. delbrueckii (35L) grown for 24 hours in a 100L fermenter in a medium containing sugarcane molasses (adjusted to 70g / L sugar), corn immersion water (25g / L) and yeast extract (10g / L) Was inoculated. Fermentation was carried out at 42 ° C. for 48 hours while maintaining pH at 6.0 using liquid ammonia and stirring at 200 rpm. Lactic acid yield was 50 g / L.

Example  4

Lactic acid fermentation was also set up in a 1000 L fermentor. The batch size was 800L. The medium composition is the same as in Example 1. L. delbreuckii inoculum (80 L) for this batch was grown in a 100 L fermentor. The fermentation was carried out at 42 ° C. for 48 hours with stirring at 200 rpm. Liquid ammonia was used to maintain the pH of the medium at 6.0. After 48 hours of fermentation, the lactic acid yield was 55 g / L.

Example  5

After centrifugation and the pH lowered to 2.0, 40 mL of fermented broth obtained by fermentation of sugars present in molasses by Lactobacillus delbruckii was determined to be 5.8% (w / v) lactic acid, 4% total sugar and total dissolved as measured by colorimetric system. It contained 20% solids, which was solvent extracted with isoamyl alcohol in a volume ratio of 1: 2 with stirring on a rotary shaker for 30 minutes. This fermentation broth contained various metal ions at different concentrations. The metal ion concentrations measured by ICP were as follows: titanium 1.76; Chromium 37.53; Manganese 578.30; Iron 2117.3; Cobalt 42.22; Nickel 7.62; Copper 7.038; Zinc 123.75; Aluminum 52.20; Lead 13.49; Molybdenum 1.76; Arsenic 1.17; Sodium 105.4; Potassium 129.03; Magnesium 35.07; Calcium 11.55 (all values are in ppm). Phase separated in 250 mL separatory funnel for 1 hour. After taking into account the volume change due to the mutual dissolution of the two liquid phases, the aqueous phase was extracted again with fresh alcohol at the same volume ratio. After five such liquid-liquid extractions, the raffinate was centrifuged at 7000 rpm for 15 minutes to remove the exported organic phase and analyzed for lactic acid. From the mass balance it was estimated that the recovery of lactic acid in the solvent was 94% and the removal of sugar was at least 90%.

Example  6

10 mL of fermentation broth, pH 2, similar to that described in Example 5, was mixed with 100 mL of diisopropyl ether (DIE) in a 250 mL conical flask and placed on a rotary shaker for 1 hour. Emulsion formation was observed during the extraction process, which was phase separated by centrifugation at 7000 rpm. Continuous four-step repeated extraction of the aqueous phase with DIE yielded a substantially colorless combined organic phase, which nearly eliminated color removal (> 99%) as the lactic acid concentration decreased from 5.8% to 1.8%. Indicates.

Example  7

50 mL of fermented broth at pH 2 with increased lactic acid concentration (7.7 wt%) was liquid / liquid extracted with 100 mL of cyclohexanone in a 250 mL volumetric funnel. The mixture was thoroughly mixed for 1 hour and left to stand. After separation of the two phases, the lower aqueous phase was twice more liquid / liquid extracted. After 3-step extraction, the raffinate had 0.8 wt% lactic acid, which showed a recovery of 90%. Extraction was both advantageous in terms of removing sugar and color from the fermentation broth. Compared to 0.23 and 5.6 for fermentation broth, the organic layer after the first extraction had a sugar-to-lactic acid ratio of 0 and a color-to-lactic acid ratio of 0.65.

Example  8

Fermentation broth used in Example 5 was used to test 1-butyl alcohol for fermentation efficiency with respect to lactic acid. The juicy-to-extractant ratio used was 1: 0.73 by weight. The mixture was placed on a rotary shaker for 1 hour and allowed to stand for an additional hour under dormant conditions to allow phase separation to occur. The extract phase thus obtained was further extracted with 1N NaOH (volume ratio 1: 2, mixed for 1 hour on a rotary shaker), wherein the alkaline phase contained 16.2 g / L sodium lactate.

Example  9

In the experiment, a rotary shaker was used to contact 46% of the weight of the fermentation broth used in Example 3 with an extractant consisting of 150 mL of ethyl ester of ethane in a capped 500 mL glass bottle for 1 hour, and then the contents Were transferred to a 500 mL separatory funnel and left alone to allow phase separation. The raffinate was contacted again at the same volume ratio as the fresh extractant. After three consecutive extractions, the aqueous phase consumed showed a decrease in lactic acid concentration and the final level was only 21% of the initial value. While the juice was dark brown in color, the combined extracts showed little color. After excluding the dilution effect, about 98% of the color was retained by the raffinate.

Example  10

Fermentation broth with a lactic acid content of 72 g / L was used to investigate the pH effect on the extraction efficiency with 1-octanol. In the first case, the juice of pH 6.2 was used as it was, the pH of the second juicy sample was adjusted to 3.5, and the pH of the third was adjusted to 3.0. In all cases, 5 mL of broth was contacted with 10 mL of alcohol, mixed by hand for 30 minutes, and allowed to stand for a suitable period to phase separate. Lactic acid in raffinate was measured to confirm that the recovery rates of lactic acid corresponding to the pre-extraction of pH 6.2, 3.5 and 3.0 were <1%, 22% and 44%, respectively. As such, lower pH values enhance the extraction efficiency to a significant extent.

Example  11

The fermentation broth at pH 2 containing 56 g / L lactic acid was extracted a suitable number of times with 1-butanol to produce a rich organic extract with 1.6 g of lactic acid per 100 mL of solvent. 32 mL of this organic extract was contacted with 8 mL of aqueous ammonia containing 3% (w / v) NH ₃ and mixed for 30 minutes on a shaker. The contents were allowed to stand for 1 hour to phase separate, and the lactic acid of the aqueous layer was measured with a colorimeter. The aqueous layer was found to contain 55.7 g / L lactic acid, indicating 87% recovery. Another 32 mL of the solvent containing lactic acid was extracted using 8 mL of 4% (w / v) aqueous ammonia under the conditions indicated above. The aqueous phase was found to have 60.9 g / L lactic acid, ie 95% recovery.

In a similar manner, the fermented broth used above was extracted an appropriate number of times with alcohol to obtain isoamyl alcohol containing 1.58% (w / v) lactic acid. 32 mL of this extract was back-extracted with 6% (w / v) aqueous ammonia in a volume ratio of 4: 1 (30 min mixing, 1 hour standing). The lactic acid concentration of the aqueous layer was 63.1 g / L, indicating that the efficiency of the back-extraction step is at least 98%.

Example  12

The concentrated butanol extract containing 18.1 g / L lactic acid obtained by extracting the fermentation broth (as used in Example 7) with 1-butanol was back-extracted with an aqueous calcium hydroxide suspension. A suspension of 2.5 g of calcium hydroxide suspended in 20 mL water was added to 200 mL of the organic extract, and the mixture was mechanically stirred in the flask for 1 hour. After standing for 1 hour the aqueous phase was separated with some solid. A total of 20 flasks were similarly treated to complete back-extraction for 4 L of rich butanol extract. The aqueous phases were combined and the pH lowered to 2-2.1 with concentrated sulfuric acid. Filtration to remove precipitated calcium sulfate; At this stage the volume was 486 mL. The lactic acid concentration in the filtrate was analyzed to give a value of 141.5 g / L, which corresponds to 95% back-extraction efficiency.

Example  13

450 L of fermented broth containing 45 mg / mL of lactic acid with an L / D ratio of 96: 4 was continuously contacted with the solvent in a rotary disk contactor. Extraction solvent is introduced at the bottom of the unit while fermentation broth is fed from the top. After phase separation, the extract phase was collected continuously from the top of the extractor and the raffinate from the bottom. The flow rates of the two phases varied in the range of 10-50 Kg / hour. After reaching the steady state, under flow conditions of 10 Kg / hour for fermentation broth and 30 Kg / hour for solvent, the concentration of lactic acid in the extractant was found to be 1.6 wt%, indicating a recovery of 95% or more.

Example  14

1260 Kg of the solvent extract containing 1.6 wt% lactic acid obtained from the fermentation broth as described in Example 13 was mixed with 26 Kg of lime dissolved / dispersed in 156 Kg of demineralized water in a 3000 L continuous stirring tank reactor (CSTR) for 2 hours, and the reaction mixture was Allow 2 hours for phase separation to occur. The lower aqueous phase contained mostly lactic acid, and the upper regenerated solvent phase was taken to lactic acid liquid-liquid extraction from fresh gravy. An aqueous lactate phase of 204 Kg was obtained from the bottom of the reactor, which exhibited at least 99% lactic acid recovery based on mass balance. The aqueous back-extract thus obtained was acidified to pH 2-2.5 using concentrated sulfuric acid in a continuous stirred tank reactor with glass inside to convert lactate to lactic acid. The acidified back-extract was then centrifuged to remove precipitated calcium sulfate. The precipitate was washed with 30 Kg of demineralized water to recover the lactic acid taken out. The filtrate containing lactic acid thus obtained was then passed through a column containing the cation-exchange resin at a rate of 3-5 fill volumes per hour under continuous column operating conditions. After this treatment the pH of the column effluent was found to decrease to a level of 1.5-2.0. In order to recover residual lactic acid, the resin column was rinsed with two packed volumes of demineralized water to completely recover the lactic acid captured from the resin column. 275 Kg of the dark brown aqueous lactic acid solution thus obtained was concentrated in a glass distillation column filled with a reflux ratio of 3: 1 through a series of condensers at a vacuum of 350 mmHg at 80 ° C to 84 ° C. This distilled water (245 Kg) was used in the back-extraction procedure with another solvent extract. 30.5 Kg of an aqueous lactic acid concentrate with 58 wt% lactic acid was obtained and the total lactic acid recovery was 88% in the process step from back-extraction to concentration.

Example  15

82 g of an aqueous lactic acid concentrate having a concentration of 66wt% lactic acid obtained from fermented broth by liquid-liquid extraction, back-extraction of lime lime, acidification of lactate, resin treatment and concentration described in the above embodiment was prepared, It was placed in a 250 mL three-neck round bottom glass flask equipped with a facility to flush nitrogen through a standard joint. The flask was connected via a short pad distillation head and a water / air condenser to a 100 mL two-neck round bottom glass flask, the collection vessel. A thermometer pocket was provided on the distillation head to measure the steam temperature. The reaction flask was placed in a rota-mantle and the reaction mixture was heated and mixed. The temperature was gradually increased to 140 ° C. while flushing nitrogen to remove water as distillate. The reaction was then cooled to 90 ° C. and then 0.6 g of dibutyltin oxide catalyst was added to the reaction mixture under nitrogen blanketing. Then, the temperature was gradually increased gradually to 160 ° C. over 2 hours and the vacuum was gradually increased gradually to 60 mmHg at atmospheric pressure to remove the remaining water. After removing the water, the reaction mixture is cooled to 90 ° C., the water condenser is exchanged with an air condenser, the collector is cooled with ice, and the temperature is gradually increased to 290 ° C. and the vacuum is gradually increased to 4 mm Hg over 5 hours. Tide was distilled off. As a product 40 g of crude lactide distillate were obtained, with the result that the yield of crude lactide was at least 90% based on lactic acid.

Example  16

83 g of aqueous lactide, a dark brown concentrate containing 75% concentration of lactic acid obtained from fermentation broth by liquid-liquid extraction, back-extraction with ammonia, ion-exchange and concentration, was prepared using the experimental setup described in Example 15. Used for tide formation. Water was removed by gradually increasing the temperature to 156 ° C. at ambient temperature over 3 hours under a nitrogen atmosphere. Cool the reaction mixture to 85 ° C., add 1.6 g of dibutyltin oxide catalyst to it, gradually increase the temperature to 160 ° C. over 4 hours, and gradually increase the vacuum to 40 mm Hg at atmospheric pressure, leaving the reaction mixture Water was removed. The reaction mixture was cooled to 90 ° C., using a short pad air condenser and cooling the product collection vessel with dry ice for lactide, starting the lactated distillation by increasing the temperature stepwise to 235 ° C. and the vacuum to 5 mm Hg. . 47 g of crude lactide distillate were collected within 4 hours, and 7.5 g of residue remained in the reaction flask, indicating a crude lactide yield of at least 90%.

Example  17

In experiments on lactide formation, 42.5 Kg of a blackish colored viscous aqueous lactic acid concentrate containing 44.7% by weight of lactic acid, prepared from fermentation broth by liquid-liquid extraction, back-extraction, acidification, resin treatment and concentration, It was used as feedstock in a SS-316 autoclave reactor with a working volume of 50L. The reactor was equipped with a turbine-type stirring impeller, a shot pad condenser, a liquid and vapor temperature measurement facility, and a port for nitrogen blanketing. A glass 20L three-necked product collection vessel with a valve at the bottom was connected to the reaction system via a condenser. The reactor was wrapped in a jacket to circulate hot oil to regulate temperature, and a vacuum was applied through one of the necks of the collection vessel using a reflux condenser. After charging the feedstock to the reactor, the temperature of the reaction mixture was slowly increased from ambient temperature to 152 ° C. over 10 hours to remove water. The steam temperature varied in the range of 100-108 ° C. during this period. Next, 340 g of dibutyltin oxide catalyst was added, the temperature was maintained in the range of 150 ° C to 160 ° C over 8 hours, and the vacuum was gradually increased from 600 to 60 mmHg to further remove water. After removing the water, a vacuum of 10 mmHg was applied, and the lactide was distilled off by gradually increasing the temperature from 160 ° C to 220 ° C over 14 hours. A crude lactide distillate of 13.4 Kg was obtained, with the result that the yield of crude lactide was 87% based on lactic acid.

Example  18

Crude lactide as prepared in Example 15 from fermentation broth was dissolved in ethyl acetate using a crude lactide: ethyl acetate ratio of 5: 3 by dissolving the lactide in a 100 mL round bottom flask equipped with a reflux condenser. Was purified by crystallization from the solution, and the solution was cooled to a temperature of 5 ° C. for 2 to 4 hours to cause crystallization of lactide. White lactide crystals having a melting point of 89 ° C. were obtained. This purified lactide was second recrystallized with ethyl acetate to produce purified lactide with a melting point of 97 ° C. comparable to that reported in the literature. 2 g of this purified lactide was taken in a 100 mL round bottom flask and evacuated in an oil bath at 80 ° C. for 1 hour, and then 0.2 mL of a 0.5% solution of tin octoate catalyst dissolved in toluene was added, and the reaction mixture was further evacuated for 1 hour to remove the solvent. Removed it. The system was sealed in vacuo and the temperature of the reaction mixture was gradually increased gradually to 180 ° C. while stirring with a magnetic bar, and held at 180 ° C. for 45 minutes to perform melt polymerization of lactide. Next, the temperature was lowered to 130 ° C. and the polymerization reaction was continued for 12 hours. The polymer thus produced was dissolved in chloroform, reprecipitated with methanol, and reprocessed to dry to obtain 1.87 g of a white product having a solution specific viscosity of 0.21, which yields 94%. DSC analysis of the polymer at a heating rate of 10 ° C./min revealed three transition properties of PLA at 55 ° C., 99 ° C. and 152 ° C., corresponding to glass transition, crystallization and melting of the polymer, respectively.

Example  19

2 g of purified lactide prepared according to the procedure described in Example 18 was taken in a 100 mL round bottom flask and evacuated at 80 ° C. (oil bath) for 1 h. Next, 0.4 mL of a 0.5% tin octoate solution dissolved in toluene was added, and the flask was evacuated for 30 minutes to remove toluene. Nitrogen was flushed repeatedly into the reaction mixture and the flask was sealed in the presence of nitrogen. The temperature of the reaction mixture was then slowly raised to 180 ° C. with stirring of the magnetic bar, allowing melt polymerization to occur for 2 hours at 180 ° C. followed by 4 hours at 150 ° C. and 13 hours at 120 ° C. The GPC molecular weight of the retreated polymer was found to be 38,700 and the molecular weight distribution was 1.98. DSC thermograms showed a melting endotherm of the polymer at 152 ° C. and a crystallization temperature of 108 ° C. during cooling.

Example  20

Crude lactide prepared from lactic acid obtained from fermentation broth as described in Example 16 was purified by recrystallization twice in ethyl acetate at a lactide: ethyl acetate ratio of 5: 3. The lactide was then filtered and recrystallized using toluene at a 1: 1 lactide: toluene ratio. The melting point of this purified lactide was found to be 99 ° C. 7.5 g of this lactide was taken in a round bottom flask and dried at 80 ° C. for 1 hour in a vacuum of 5-10 mmHg. Next, 0.6% tin octoate catalyst based on lactide was added and the reaction mixture was further dried under vacuum for 1 hour. Next, the flask was sealed under vacuum and the reaction mixture was polymerized at 180 ° C. for 35 minutes. A polymer having an inherent viscosity of 0.69 dL / g and a GPC molecular weight of 78,000 was obtained with a yield of 90%.

Example  21

Melt polymerization of 10 g of purified lactide having an acidity of 0.01 wt% and a water level of 180 ppm as lactic acid was performed at 180 ° C. for 3 hours. 0.2% tin octoate catalyst based on lactide weight was used. The experimental setup for melt polymerization was the same as described in Example 20. A polymer having an inherent viscosity of 2.52 dL / g and a GPC molecular weight of 2,35,000 was obtained with a polymer yield of 95%. DSC analysis of the polymer sample showed a melting point of 170 ° C. and a heat of fusion of 29 Kcal / mol.

Example  22

Melt polymerization of the purified lactide was performed at 100 g per batch level. 100 g of lactide with an acidity of 0.02% and water of 350 ppm were taken in a 250 mL round bottom flask. 0.056 wt% tin octoate catalyst based on lactide was added and the reaction mixture was evacuated at 80 ° C. for 2 hours to remove water and toluene used to prepare the catalyst solution. The reaction mixture was sealed under vacuum and the temperature was increased to 180 ° C. and maintained for 1 hour to allow melt polymerization to occur. A polymer having a GPC molecular weight of 2,13,000 and a dispersion degree of 2.12 was obtained with a conversion rate of 85%.

Example  23

1.3Kg of purified lactide was melt polymerized in the presence of 0.056% tin octoate catalyst in a 5Kg Paar reactor equipped with a mechanical stirrer. The acid value of lactide was 0.25%. Lactide was dried at 75-85 ° C. for 4 hours at a vacuum level of 20-30 mmHg. Nitrogen was repeatedly purged into the mixture and the assembly was sealed under vacuum. The temperature of the reaction mixture was slowly raised to 180 ° C. with stirring and maintained for 1 hour. 1.1 Kg of polymer having a specific viscosity of 0.77 and a GPC molecular weight of 72,000 was obtained with a polymer yield of almost 85%.

In view of the foregoing description, it will be apparent to those skilled in the art that equivalent variations thereof may be made without departing from the spirit and scope of the present invention.

Claims (29)

Preparing a fermentation medium having molasses as a carbon source; Fermenting the fermentation medium; Extracting lactic acid from the fermentation medium; Removing the pigment from the lactic acid; Purifying and concentrating the lactic acid; Preparing lactide from the lactic acid; And Polymerizing the lactide to form polylactic acid A method for producing polylactic acid by fermentation of renewable agricultural raw materials, which can avoid waste disposal problems, including. The method of claim 1 wherein the renewable agricultural raw material is selected from the group consisting of molasses, sugar cane bagasse hydrolyzate, corn syrup, sugar beet and sugar cane juice. The method of claim 1 wherein the renewable agricultural raw material can be edible or non-edible. The method of claim 1 wherein the renewable agricultural raw material is sugarcane molasses containing from about 40% to about 50% (w / w) fermentable sugars. 2. The method of claim 1, wherein said molasses is adjusted to a fermentable sugar concentration of about 70 g / L. 6. The method of claim 5, wherein the molasses is centrifuged to remove suspended material and diluted with water to produce a fermentable sugar concentration of about 7% to about 10%. 7. The method of claim 6, further comprising supplementing the molasses solution with about 2% to about 5% corn steep water and autoclaved yeast paste. The method of claim 1 wherein the fermentation step is carried out under anaerobic / aerobic conditions. 9. The method of claim 8, wherein said fermentation under anaerobic / aerobic conditions is carried out by a microorganism selected from the group consisting of Lactobacillus, Streptococcus, Bacillus and Rhizopus . The method of claim 9, wherein Lactobacillus is selected from the group consisting of L. delbrueckii , L. rhamnosus , L. helveticus, L. casei , L. plantarum , L. bulgaricus , L. amylovorans and L. lactis . How to. The method of claim 1, wherein the fermentation step further comprises adjusting and maintaining the pH of the fermentation medium to about 5.0 to about 6.0, thereby avoiding growth retardation of the microorganisms. The method of claim 1, wherein the fermentation step is a continuous process or a fed-batch fermentation process. 12. The method of claim 11, wherein said adjusting said pH of said fermentation medium is accomplished using a neutralizing agent selected from the group consisting of alkalis, carbonates, and ammonia. The method of claim 1, wherein the lactic acid produced in the fermentation broth is extracted using an organic solvent. 15. The method of claim 14, wherein said extracting step uses an organic solvent selected from the group consisting of isoamyl alcohol, butanol, cyclohexanone and methyl acetate. The method of claim 1, wherein the extracting step further comprises separating the lactic acid from the fermentation medium at ambient temperature using a solvent. The method of claim 15, wherein the solvent is an alcohol or an ester. The method of claim 1 wherein the method has an extraction efficiency of at least 90%. The method of claim 1, wherein said extracting step is not energy intensive. 15. The method of claim 14, wherein the extracted lactide is purified and concentrated. 21. The method of claim 20, wherein the purification comprises back-extracting lactic acid from the organic solvent into the aqueous phase using an alkali hydroxide or alkaline earth metal hydroxide. 22. The method of claim 21, wherein the lactic acid back-extracted with the aqueous phase is acidified to a pH in the range of 2 to 2.5 using concentrated sulfuric acid. The method of claim 1, wherein the purification comprises passing the back-extracted lactic acid of claim 22 through a cation-exchange resin. 24. The process of claim 23, wherein the purified lactic acid is concentrated in a packed distillation column of glass interior. The method of claim 24, wherein the concentrated lactic acid is converted to lactide in the reactor at a temperature of about 120 ° C. to about 155 ° C., wherein the corresponding liquid temperature is about 180 ° C. to about 225 ° C. 26. The method of claim 25, wherein the concentration of lactic acid to lactide is performed in a vacuum of about 5 to about 30 mmHg, wherein prepolymerization and lactide formation are combined in a single operation. The method of claim 1 wherein the purification of lactide comprises crystallizing using an organic solvent. 28. The method of claim 27, wherein the purified lactide is polymerized to obtain polylactic acid. 5. The method of claim 4, wherein the renewable agricultural raw material comprises sugarcane bagasse, the sugarcane bagasse hydrolyzes the bagasse particles with dilute acid at 120 ° C. for 30 to 90 minutes, A process characterized in that it is prepared by delignification by treatment with 1% to 5% (v / v) alkali at 120 ° C. to 150 ° C. for about 15 to about 90 minutes.

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